The invention relates in general to machining and in particular to the electrical machining or drilling of holes.
Rotating cutting drills may be used to drill blind holes or to enlarge existing holes. The aspect ratio of a hole is the ratio of the length or depth of the hole to the diameter of the hole. As the aspect ratio of a hole becomes large, removal of chips (machined off of the work piece) may become a principal concern.
One method to improve precision on rotating drills may be to focus upon achieving a stiff cutting tool, or employing mechanical guides within the hole to maintain precision. A second approach may be to incorporate an active guide within the work piece. The active guide may further complicate chip removal and may be challenging to employ for very small diameter holes.
Energy beam machining methods may include laser-beam machining and electron-beam machining. These methods may be used to drill a line of sight hole. These methods are generally limited to shallow holes due in part to debris interfering with the line of sight path as the work piece material is removed by the energy beam.
Certain applications may present challenges for known drilling methods. For example, using known methods, it may not be viable to drill a 12 mm diameter hole that is 6 meters deep, with a blind hole aspect ratio of 500. Holes of this type may be used, for example, as cooling channels in railguns. Because of the difficulty in drilling such holes, railgun designs may use welded or mechanically bound rails to allow cooling channels to be machined prior to joining the parts (K. Jamison et al., “Thermal Loading and Heat Removal from a Sequentially Fired Railgun, IEEE Transactions on Magnetics, vol. 31, no. 1, pp. 314-319, January 1995). High aspect ratio holes may also be used to route power, sensor, data, or telecommunication transmission lines through structural elements.
Challenges for known drilling methods may include: 1) the transmission of rotating mechanical cutting power through a long slender shaft; 2) the loss of cutting tool directional control through a long slender shaft; 3) work piece material removal through a long slender hole; and 4) the interaction of mechanical cutting loads and related disturbances upon the application of control forces.
Various electrical machining methods are known. Electrical machining methods may include electrochemical machining (ECM), shaped-tube electrolytic machining (STEM), electro-stream (ES), electrical discharge machining (EDM), and electrical discharge wire cutting (EDWC). G. Bellows and J. Kohls (“Drilling without Drills” Special Report 743, American Machinist, March 1982) review some of the known electrical machining processes. Bellows and Kohls' Special Report 743 is expressly incorporated by reference herein.
FIG. 1 is a schematic diagram of an example of a known electrical machining process. A hole 10, for example, may be electrically machined in a work piece 12. An electrical machining tool 14 may be supported and controlled by a tool holder 16. A controller 19 may be included as part of tool holder 16, or may be a separate component. Tool 14 may have an electrically conductive, tubular portion 15 with a hollow interior 24. A power supply 18 may be connected between tool holder 16 and work piece 12. Tool 14 may function as an electrode, such as a cathode, and work piece 12 may function as an electrode, such as an anode.
Hole 10 generally includes an overcut. The overcut results in hole 10 having a cross-section that is larger than the cross-section of tubular portion 15. Thus, entire tool 14 may be inserted in hole 10 with sufficient annular clearance between tool 14 and hole 10. An electrical machining fluid (EMF) 20 may circulate from tank 22 through an EMF processor 26 to tool holder 16, through hollow interior 24 of tool 14 into hole 10 in work piece 12, and then back into tank 22, as shown by the arrows in FIG. 1. Processor 26 may process EMF 20 by, for example, filtering, removing sludge, temperature conditioning, etc.
In the case of ECM, EMF 20 may be an electrolytic fluid and hole 10 in work piece 12 may be created by anodic dissolution. In the case of EDM, EMF 20 may be a dielectric fluid and hole 10 in work piece 12 may be created by erosive dissolution caused by spark discharges. In the case of electrochemical discharge machining (ECDM), EMF 20 may be a semidielectric fluid and hole 10 in work piece 12 may be created by a combination of anodic and erosive dissolution. A discussion of ECDM appears in Coteat{hacek over (a)} et al, “Electrochemical discharge machining of small diameter holes,” Engineering International Journal of Material Forming, Volume 1, Supplement 1, pp. 1327-1330, 2008, which is expressly incorporated by reference herein.
For use in ECM, tool 14 may be manufactured with a layer of electrically insulating material 28 around conductive tube 15. Of course, at the head 17 of tool 14 at least a portion of tube 15 must not be covered with insulating material 28 so that an electric circuit may be established between tube 15 and work piece 10. Insulating material 28 may prevent unwanted anodic dissolution of sides of hole 10. In EDM or ECDM, insulating material 28 may not be required around tube 15.
In electrical machining processes, tool holder 16 may provide one of more of the following functions: 1) axial translation (feed) of tool 14; 2) rotation of tool 14 about its longitudinal axis; 3) distribution of electrical current to tool 14 for anodic and/or erosive dissolution of work piece 10; and 4) channeling of EMF 20 to tool 14. See, for example, U.S. Pat. No. 3,590,204 issued to O'Connor on Jun. 29, 1971.
As hole 10 machined in work piece 12 increases in depth, the side walls of hole 10 may guide tool 14 on its intended path through work piece 12. Accurately drilled holes may be limited in their depth by the compliance and straightness of tool 14. As the aspect ratio of hole 10 becomes large, the propensity of tool 14 to bend may increase, thereby causing a loss of precision in drilling.
For example, if a simple round tool of a given diameter is considered a cantilevered beam, the tip deflection of the tool for a given lateral disturbance may increase as the third power of the tool length (F. Beer and E. R. Johnston, Jr., “Mechanics of Materials,” McGraw-Hill 1981). Similarly, torsional stiffness of a simple cantilevered tool is inversely proportional to length. Rotational deflection at the cantilever tip, for a given torsional disturbance, may increase in proportion to the tool length (Beer and Johnston, supra).
A known ECM process (FIG. 2) may be used to machine a hole 30 in a work piece 34 having a pilot hole 32. An ECM tool 36 may include a solid conductive cylinder 38 covered with an insulating layer 40. As noted before, at the head 42 of tool 36 at least a portion of cylinder 38 must not be covered with insulating material 40 so that an electric circuit may be established between cylinder 38 and work piece 34. Pilot hole 32 and the annular area 44 between tool 36 and work piece 34 may provide a flow path for EMF 20. The direction of flow may be either into or out of pilot hole 32.
A process akin to that of FIG. 2 was used to enlarge a pilot bore and create a naval cannon with a five inch caliber (inner diameter) and a length of 270 inches. In that case, the straightness of the pilot bore was relied upon to achieve the desired final gun bore centerline (Wessel, “Electrochemical Machining of Gun Barrel Bores and Rifling,” Naval Ordnance Station, Report No. MT-050, Louisville, Ky., September 1978).
A need exists for a more accurate method of electrically machining large aspect ratio holes.